1. Field of the Invention
The present invention relates to a radiography apparatus for performing radiography to generate slice images of a subject.
2. Description of the Related Art
In recent years, tomosynthesis radiography was proposed to observe affected regions of patients in more detail by a radiography apparatus using X-rays. In tomosynthesis radiography, radiography is performed by moving an X-ray tube to different positions, and by irradiating a subject by the X-ray tube at different angles from the different positions. Further, images obtained by radiography are added to obtain an image in which a desired cross section of the subject is emphasized. In tomosynthesis radiography, the X-ray tube is moved parallel to an X-ray detector, or in such a manner to draw a circle or an ellipse or oval, based on the characteristic of a radiography apparatus and the kind of slice images (tomograms) to be obtained. The subject is radiographed at different radiation angles to obtain a plurality of radiographic images, and a slice image of the subject is reconstructed from the plurality of radiographic images. In generation of slice images by tomosynthesis radiography as described above, a slice image of the subject in which a structure on a desired cross-section of the subject is emphasized can be obtained by blurring and suppressing structures which are not on the desired cross-section. Therefore, it is possible to improve the visual recognition characteristic of images in observation of lung nodules or nodes, in which structures overlap with each other and are hard to observe, microfractures of bones, and the like.
Further, in application of tomosynthesis radiography, a method in which pre-shot radiography is performed before tomosynthesis radiography has been proposed (please refer to U.S. Pat. No. 7,292,675 (Patent Document 1)). In the pre-shot radiography, a subject is irradiated with low-dose X-rays. Further, the dose of X-rays in the tomosynthesis radiography is set by using a pre-shot image obtained in pre-shot radiography and the condition of radiography in the pre-shot radiography. Further, a method in which transmission data about a subject is obtained before tomosynthesis radiography has been proposed (please refer to Japanese Unexamined Patent Publication No. 2005-149762 (Patent Document 2)). In the method, the transmittance of the subject is measured based on the transmission data, and the tube voltage of the X-ray tube during radiography is set based on the transmittance. Further, a method in which a region for setting a region of interest is determined based on a past radiographic image of a sample (subject) has been proposed (please refer to PCT Japanese Publication No. 2003-299643 (Patent Document 3)). In the method, the region for setting the region of interest is correlated with scanograms for determining an actually imaged cross-section, and the region of interest, which is an actual range of radiography, is determined. Accordingly, the dose of radiation that passes the outside of the actual region of interest during radiography is limited.
However, Patent Document 1 merely discloses setting the dose of X-rays in tomosynthesis radiography by using a pre-shot image. Further, Patent Document 2 merely discloses setting the tube voltage of an X-ray tube. Further, in the method disclosed in Patent Document 3, it is necessary to refer to a past radiographic image of the subject, obtained in the past radiography, to set a region of interest.
Further, in the method disclosed in Patent Document 2, it is necessary to obtain transmission data to measure the transmittance of the subject before tomosynthesis radiography. Therefore, additional irradiation of the subject with radiation is necessary. Further, in the method disclosed in Patent Document 3, it is necessary to obtain scanograms to set the region of interest. Therefore, additional irradiation of the subject with radiation is required.
When a slice image is generated in a tomosynthesis apparatus, it is necessary to irradiate a subject with X-rays a plurality of times. Therefore, the dose of radiation irradiating the subject is larger than the dose of radiation irradiating the subject in plain radiography (simple radiography), in which an X-ray image of the subject is obtained by irradiating the subject with X-rays only once. Therefore, a plain radiographic image obtained by plain radiography may be used first to diagnose a patient, and a slice image may be generated only when it is necessary. For example, only when a suspected bone fracture cannot be confirmed by using a plain radiographic image, a slice image may be generated.